The present invention relates to a method for the topically precise subsequent-engraving of printing cylinders.
It has already, been disclosed in German Patent 23 61 903 which corresponds to U.S. Pat. No. 3,997,838 and in German Patent 25 13 042 which corresponds to U.S Pat. No. 4,057,838, both incorporated herein by reference, that parts of masters can be subsequently engraved with precise registration at very specific locations of the circumference of a printing form on a form cylinder that has already been partially engraved. This problem arises in magazine printing when, for example, a magazine page contains an advertising portion and a current portion. The advertising portion is engraved and proofed long before the magazine is issued in order to be able to submit a proof to the customer for quality control. The current portion is then subsequent-engraved shortly before the magazine is finished. It is important that the subsequent-engraving on all printing cylinders of a set of color plates is positioned with precise registration relative to the initial engraving.
Solutions have been described in the two cited patents for electro-mechanical engraving wherein an angular offset between the scanning cylinder and the engraving cylinder is measured by means of stationary reference marks on the scanning and engraving cylinders. One of these reference marks is often referred to as the north pulse because it only appears once on the circumference of the printing cylinder, and pulses are blanked out or inserted by the voltage of the drive motor for the engraving cylinder until the angular offset becomes zero. For this purpose, a raster disc connected to the printing cylinder is photo-electrically scanned and a circumference pulse--also referred to as a north pulse--is generated upon every revolution of the cylinder. The raster disc also supplies a multitude of pulses with which an engraving clock is generated and synchronized by means of a PLL circuit.
This procedure is not possible in electron beam engraving of printing cylinders because there is no scanning cylinder that rotates synchronously with the printing cylinder since the engraving is operated offline to the scanning, i.e. from a memory. As a consequence, an angular offset to the scanning cylinder that could be evaluated as a criterion for the re-phase-in cannot be identified.